Kiwi archaeologists cook up a record of Earth’s magnetism

What’s that in your hangi?

A kind of oven used by the Maori since long before Europeans arrived in New Zealand is being mined for archaeological information about the history of Earth’s magnetic fields.

Scientists from Victoria University in Wellington, New Zealand, say that there’s a gap in the global palaeomagnetic record in the south-west Pacific, and hopes that by finding and examining ancient earth ovens – “hangi” in the indigenous Maori language – the record can be filled.

The hangi follows a form common in many societies: a hole in the ground is lined with hot stones and covered with the meat to be cooked (on a wooden rack), with damp material such as fern fronds providing water to steam the meat.

Dr Gillian Turner, who is leading the work, explained to the BBC that stones used in the earth ovens could have been heated enough that magnetic particles in the stones would re-align to the Earth’s magnetic field.

Thermocouples placed in an experimental hangi showed the temperatures among the rocks reach around 1,100°C.

That’s hot enough that some rocks – specifically andesite boulders, which are a hangi favourite because they don’t shatter in the heat – re-magnetise. The rocks pass their Curie temperature, and as they cool, their magnetic fields will be aligned to Earth’s.

Hence by measuring the magnetism in abandoned pits, Dr Turner hopes to reconstruct records going back between 500 and 700 years, when the Maori first arrived. Charcoal in the ovens will be used to carbon-date the sites.

Older magnetic information can be reconstructed from sources such as volcanic rocks and lake sediments – a good thing, since Dr Turner hopes to fill in a magnetic record over about 10,000 years. ®